1. Field of the Invention
The invention relates to pallet positioners for loading and unloading packages or objects unto pallets from an elevated position; more specifically, it relates to a pallet positioner that can maintain the top of a changing load at a convenient predetermined height.
2. Description of the Prior Art
In the handling of a number of packages, boxes, or other objects, a common task is to manually transfer them between an elevated table, conveyor, shelf, or other similar location, and a pallet resting on the floor. While the shelf, for example, remains at a fixed height, the top of the load on the pallet, where the next box or object is to be loaded or removed, usually is at a different height which varies as the packages, etc., are piled on or removed from the pallet. This difference in height, and the changes in this difference during loading or unloading of the packages, can be fatiguing for the person doing the moving. Therefore, pallet positioners, also known in the art as a load elevators, have been developed for raising the pallet from the floor to a more convenient height and even for automatically adjusting the height of the pallet as the load increases or decreases, so that the preferred height of the top of the load is maintained.
U.S. Pat. No. 5,299,906 discloses a self-adjusting pallet positioner supported by a scissors linkage located under the load platform. The vertically expandable scissors linkage is coupled to an air-actuator chamber that includes a compressible bellows and a fixed-volume reservoir placed under the load platform. The bellows is compressible between specified maximum and minimum bellows heights which correspondingly determine substantially different maximum and minimum bellows volumes. The air reservoir is coupled to the bellows and has a fixed volume that is substantial compared to the difference between the maximum and minimum bellows volumes. As a result of this configuration, the pallet positioner exhibits very good self-leveling characteristics.
In the positioner""s unloaded condition the scissors linkage is extended and the platform is situated at a convenient level for loading packages and/or materials over a pallet placed on the platform. As boxes or crates are stacked on the pallet and the weight and height of the stack on the pallet increase, the scissors linkage and the air bellows automatically contract under the load and the platform sinks approximately in proportion to the height increase of the stack for uniform loads. Thus, the top of the stack is maintained at a roughly constant level by the self-leveling feature and the stacking process is facilitated.
When the pallet is fully loaded, it is removed from the platform with a fork lift or a pallet truck. At this time, the platform is in its lowest position. In order to permit removal of the loaded pallet with a pallet truck, though, the platform needs to be at or near ground level. To this end, the elevator is installed over a pit which can receive the scissors linkage and bellows in the lowermost position of the platform. While this arrangement operates satisfactorily, the need for a pit increases costs, poses potential hazards, and prevents the elevator from being readily relocated.
U.S. Pat. No. 5,782,602 describes a low-profile pallet positioner designed to overcome the need for a pit while using a pallet truck in removing the loaded pallet from the positioner""s platform. The device consists of an upright, lateral housing and a pair of spaced, parallel outriggers extending from the housing. Vertical guide rods are mounted on the exterior of the housing and a cantilevered platform assembly is movable over the outriggers up and down along the guide rods. In its lowermost position, the platform assembly sits on the ground between the outriggers, thereby permitting the placement of a pallet truck under the pallet and its removal from the platform. The cantilevered platform assembly is driven by a cylinder-and-piston unit which is disposed inside the lateral housing and engages the periphery of the assembly through a slot in a wall of the housing. The cylinder-and-piston unit is extended and retracted by a power unit consisting of a motor, pump and tank likewise disposed inside the housing. Because of the need to maintain a sufficiently low platform profile to permit pallet-truck access to the pallet when the platform is at its lowest position, no pneumatic bellows is included under the platform. Accordingly, no self-leveling mechanism is provided. Therefore, the platform is raised and lowered strictly as needed by the operation of the power unit. As such, the advantages of a self-leveling operation are lost.
The present invention is directed at providing a low-profile self-adjusting pallet positioner that permits the removal of a loaded pallet from its platform using a conventional pallet truck. The invention combines a cantilevered-platform configuration, designed to produce a low profile, with a pneumatic bellows unit placed on the side of the platform to provide self-leveling operation.
It is an object of the invention to provide a self-leveling stacking and unstacking device with a sufficiently low platform profile to permit pallet unloading with a conventional pallet truck.
It is a further objective of the invention to provide a pallet positioner with a safety mechanism for preventing sudden or jerky motion that may be dangerous to a user.
Another object is a pallet positioner with a rotatable load platform to permit easy access to all parts of the pallet from a loading or unloading station.
According to these and other objectives, the present invention includes a support frame that consists of a main upright structure with a pair of spaced, parallel outriggers extending horizontally at ground level. The distal ends of the outriggers are connected by a flat, low-profile ramp. The upright structure includes a pair of vertical guide masts rigidly mounted on a horizontal base bracing the proximal ends of the outriggers. A platform assembly consisting of a cantilevered carrier with a rotatable platform is mounted laterally in rolling engagement on the vertical guide masts for vertical motion along the upright structure. In its lowermost position, the platform assembly sits on the ground between the outriggers and the low-profile ramp. A braking system under the platform prevents its rotation at ground level.
According to one aspect of the invention, a pneumatic bellows and an air reservoir are mounted on the horizontal base between the guide masts. The bellows is adapted to raise the carrier by acting on a pulley block connected to two vertical hydraulic cylinders mounted on the horizontal base. A belt with opposite ends affixed to the carrier and the base cooperates with the pulley block to position the carrier at a vertical height corresponding to the bellows expanded state. As the load placed on the carrier increases, the bellows is compressed and the carrier lowered proportionally, such that the top of the load on the cantilevered platform remains approximately at the same height. Accordingly, the initial pressure in the bellows and air reservoir is chosen to produce the desired height adjustments as a function of the expected density of the materials to be loaded on the platform.
In accordance with another aspect of the invention, the hydraulic cylinders provide dampening that prevents oscillations when the carrier is lowered in response to incremental loads placed on the platform. The cylinders also provide a stabilizing structure to ensure the linear vertical expansion and contraction of the resilient bellows chamber in response to load changes. Further, the flow of hydraulic fluid to and from both sides of the cylinders is regulated with valves to prevent the uncontrolled upward motion of the carrier when the load is decreased and to provide a safety check before the platform is lowered to ground level. Because the self-leveling mechanism of the invention provides the upward force needed to support the load placed on the carrier, this pallet positioner does not require an independent actuating unit for raising or lowering the platform assembly.
Additional features and advantages of the invention will be forthcoming from the following detailed description of preferred embodiments when read in conjunction with the accompanying drawings.